The invention relates to a method and a device for driving at least one capacitive actuator, in particular a piezoelectrically operated fuel injection valve of an internal combustion engine.
Piezoelectric actuators contain a multiplicity of piezoceramic layers, and form a so-called stack, which upon the application of a voltage changes its dimensions, in particular its length s by a deviation ds, or generates an electric voltage in the event of a mechanical compression or tension.
Published, Non-Prosecuted German Patent Application DE 41 22 984 A1 discloses a driving device for a piezoelectric element in which the driving of the piezoelectric element is performed via corresponding electronic switches having prescribed charging and discharging times.
Published, Non-Prosecuted German Patent Application 196 32 872.1 A, corresponding to U.S. patent application Ser. No. 09/250,875, filed on Feb. 16, 1999, has already proposed a method for driving a capacitive actuator, in accordance with which the actuator is charged with a prescribed charging voltage until the voltage measured at the actuator during the driving operation reaches a prescribed value.
It is accordingly an object of the invention to provide a method and a device for driving at least one capacitive actuator which overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which a method of voltage control during driving of at least one capacitive actuator is possible even when it is impossible to break off the charging operation during driving.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for driving at least one capacitive actuator, including a piezoelectrically operated fuel injection valve of an internal combustion engine, which contains:
charging, at a start of a driving operation, an actuator of a plurality of actuators with a prescribable charging voltage via a coil from a series circuit composed of a charging capacitor and a charge-reversing capacitor, and discharging the actuator into the charge-reversing capacitor at an end of the driving operation;
comparing an actuator voltage of the actuator due to the prescribable charging voltage with a prescribed desired-value voltage;
determining a new charging voltage for a next driving operation in dependence on a difference between the prescribed desired-value voltage and the actuator voltage; and
charging the charging capacitor for the next driving operation to a voltage corresponding to a difference between the new charging voltage and a voltage present across the charge-reversing capacitor.
With the foregoing and other objects in view there is further provided, in accordance with the invention, a device for driving at least one capacitive actuator, including:
a voltage source having a positive pole and a negative pole;
a charging capacitor disposed between the positive pole and the negative pole;
a control circuit controlling the voltage source;
a first series circuit disposed parallel to the charging capacitor and containing a charging switch connected to the positive pole for conducting current away from the positive pole and a discharging switch connected to the negative pole for conducting current toward the negative pole;
a ground terminal;
a second series circuit disposed between a connection point of the charging switch and the discharging switch and the ground terminal, the second series circuit containing a charge reversing capacitor having a recharge voltage connected to the charging switch and a coil;
at least one third series circuit connected in series with the second series circuit and containing an actuator having an actuator voltage and a controlled Power-MOSFET-switch; and
a diode disposed parallel to the at least one third series circuit and conducting from the ground terminal to the coil.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for driving at least one capacitive actuator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.